1. Field of the Invention
This invention relates to an image forming apparatus using the electrophotographic method such as a copying apparatus, a printer or a facsimile apparatus.
2. Related Background Art
Image forming apparatuses according to the earlier technology use various methods such as the electrophotographic method, the thermal transfer method and the ink jet method. Among these, the electrophotographic method is excellent as compared with the other methods in terms of high speed, high quality of image and quietude, and has widely spread in recent years.
A color image forming apparatus using this electrophotographic method is grouped into various methods such as the multi-development method in which color images are superposed on the surface of a photosensitive member, whereafter they are collectively transferred to thereby effect image formation, the multi-transfer method in which the cycle of development-transfer is repetitively effected, and the intermediate transfer method in which developed images of respective colors are once sequentially transferred onto an intermediate transfer member, and thereafter are collectively transferred to a transfer material, among these, particularly the intermediate transfer method is an excellent method for the reasons that there is no possibility of colors being mixed with one another and that application to various media is possible.
FIG. 5 of the accompanying drawings shows an example of the construction of the intermediate transfer belt method which is an intermediate transfer method. In FIG. 5, a photosensitive drum 101 is rotated in the direction indicated by the arrow, and around the peripheral surface thereof, there are disposed a plurality of developing devices, i.e., a black developing device 105, a magenta developing device 106, a cyan developing device 107 and a yellow developing device 108, which are designed to be brought into contact with the photosensitive drum 101 by means, not shown, as required.
The photosensitive drum 101 is uniformly charged by a charger 102, and a latent image is formed thereon by a laser beam 104 with the aid of a laser exposure optical system 103 or the like. Next, this latent image is developed by one of the aforementioned developing devices 105 to 108, and is sequentially primary-transferred onto an intermediate transfer belt 109 rotated in the direction indicated by the arrow in a primary transfer portion (transfer position). The above-described steps are also successively effected with respect to the other developing devices, and color images of four colors superposed on top of one another are formed on the intermediate transfer belt 109, whereupon a secondary transfer roller 111 is brought into contact with the intermediate transfer belt with a transfer material 118 interposed therebetween, and the color images are collectively secondary-transferred onto the transfer material 118.
At the above-described primary transfer step, if the photosensitive drum 101 is, for example, an OPC (organic photoconductor) photosensitive member of the negative polarity, toners of the negative polarity are used when the exposed portion exposed by the laser beam 104 is developed by the developing devices 105 to 108, and accordingly, a transfer bias of the positive polarity is applied from a bias voltage source 120 to the primary transfer roller 110, and transfer is effected.
However, when in the primary transfer portion of the above-described image forming apparatus, the peripheral speeds of the photosensitive drum 101 and the intermediate transfer belt 109 are equal to each other, there has been the problem that sufficient transfer efficiency is not obtained and a so-called hollow image (edge effect) is created.